Polyisocyanates of isocyanurate structure are obtained by the trimerization of organic diisocyanates (cf. German Patents Nos. 951,168; 1,013,869; 1,112,285; 1,022,789 and 1,203,792; British Pat. Nos. 809,809; 821,158; 837,120; 856,372; 972,173; 920,080; 952,931; 944,309; 954,095; 962,689; 966,338; 941,379 and 949,253; U.S. Pat. Nos. 3,154,522 and 2,801,244; French Patent No. 1,510,342; and Belgian Patent No. 718,994).
Depending upon the degree of trimerization (percentage of the isocyanate groups in the diisocyanate used as starting material which are trimerized), the polyisocyanate mixtures of isocyanurate structure obtainable by this process contain, in addition to tris-isocyanato-isocyanurates, variable amounts of unreacted starting diisocyanate and higher polyisocyanates, i.e. polyisocyanates containing several isocyanurate rings. Thus, it is generally necessary, in order to minimize the content of free monomeric diisocyanate, to trimerize more than 60% of the isocyanate groups present in the diisocyanate used as starting material. This leads to the relatively high molecular weight products just mentioned, the properties of which are inadequate for numerous applications. Polyisocyanate mixtures of this type having an increased content of relatively high molecular weight polyisocyanato-polyisocyanurates are only suitable to a limited extent, for example, as cross-linkers for adhesives or elastomers, as polyisocyanate component in the production of polyurethane foams or as hardeners for low-solvent polyurethane lacquers.
The application of keeping the degree of trimerization below 60%, and preferably below 50%, in order to avoid the formation of relatively high molecular weight products, so that the excess diisocyanate used as starting material may subsequently be separated from the end product, is accompanied by considerably practical difficulties. Despite elaborate installations, the high melting point of polyisocyanates containing isocyanurate groups, the pronounced tendency thereof towards solidification or crystallization and the high viscosity of the melts tend to combine to make the distillation installations highly unreliable in operation. The outcome is thermal overstressing and, hence, decomposition and polymerization of the polyisocyanurate polyisocyanates.